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What makes a mare a difficult breeder? (Proceedings)

Horse breeding is the procedure by which an adequate number of sperm are deposited into the uterus of a "healthy" mare at the right time. Although semen deposition is performed in all breeds by natural cover (NC), it is often, and with increasing frequency done artificially. Although the main difference between natural breeding and artificial insemination (AI) is the delivery method of semen into the uterus, many factors determine the success or failure of the breeding. Natural breeding, if performed properly, should not be considered a cheap alternative to artificial insemination. Many horse breeders are of the opinion that if the mare does not become pregnant by AI, turning her out with a stallion to be pasture bred or natural mating will maximize the chances of establishing a pregnancy. In my opinion, a mare that fails to become pregnant by AI with good semen quality and does so by natural cover without human intervention, is an example of poor or inadequate breeding management by the personnel performing the AI. Our ability to differentiate between physiologic and pathologic conditions and to establish appropriate therapies is a hallmark of those of us that practice reproductive medicine, and should be the basis for proper reproductive management.

Breeding management includes: 1) Diagnostic procedures to determine the soundness for breeding of both male and female, 2) Necessary therapies pre and post breeding, and 3) Determination of the optimal time for insemination. This paper will discuss managerial procedures as well as address some common problems that veterinarians encounter when breeding mares by artificial insemination.

Stallion diagnostic procedures

Before deciding what stallion to breed a particular mare, it is important to determine whether the semen will be fresh, cooled or frozen. If cooled semen is chosen, collection and shipping schedules, as well as sperm longevity and stallion fertility need to be determined. If the mare is to be bred with frozen semen, semen quality after thawing as well as historical fertility of the frozen semen should be established. If possible, one should work with experienced laboratories that have a good reputation for freezing stallion sperm. If no information about the semen can be obtained, one may assume that the semen will be of mediocre to poor quality. Unfortunately, mare owners spend weeks, months or even years settling on a stallion that "best suits" their mare with little or no regard for semen quality. Because of this habit, veterinarians must educate owners on the pitfalls of using cooled or frozen semen, explain the added costs of these techniques and clearly present realistic pregnancy rates per attempt.

Due to the processing of semen, some of the heterogeneity of the raw ejaculate is lost, and this relatively physiologically homogeneous sample, coupled with the reduction in sperm numbers requires that the semen be deposited in the uterus close to the time of ovulation. Fertility of cooled and frozen semen from stallions ranges greatly,1,2 although the quality of frozen semen has been improving steadily over the last few years. The veterinarian serving the mare owner rarely has control of the quality of the shipped cooled or frozen semen so the breeding management prior to and after insemination is the most important tool that a veterinarian has to establish a pregnancy in a given mare.

Mare diagnostic procedures

Age and reproductive history are probably the most important pieces of information that a veterinarian needs when examining a mare. Many mare owners unrealistically expect that that their mare should become pregnant at the first cycle regardless of her age or reproductive history. These expectations are extremely common when the mare has had a long and successful performance career, is 10 years or older and is still a maiden. Clinical findings common to the older maiden mare include accumulation of uterine fluid during estrus, wide spread endometrial glandular dilation, anovulatory hemorrhagic follicles and cervical incompetence resulting in a failure to relax either due to fibrotic changes or adhesions.

Mares in their mid-teen years start to have a reduction in their fertility potential and therefore must be considered potential candidates for repeat breeding.

So how should we define a problem breeder? I consider that a mare is a problem breeder when she has been bred in two consecutive cycles with good quality semen and at the appropriate time. Signs that a mare is a candidate for potential problems could include one or all of the following: 1) Irregular inter-ovulatory intervals (too long or too short), 2) Presence of free fluid in the uterus before and/or after insemination, 3) Increased uterine edema after insemination, or 4) Presence or persistence of marked endometrial edema post-ovulation.